Crustacean antimicrobial peptides

ABSTRACT

The invention concerns antimicrobial peptides obtained from penaeid prawns having the following characteristics: a molecular mass of about 5 to 7 kDa; a pHi not less than 9; an N-terminal portion comprising a region (A) of about 15 to 25 amino acids rich in proline; and a C-terminal portion comprising a region (B) of about 20 to 30 amino acids and containing 6 cysteine residues forming three intramolecular disulfide bonds. The invention also concerns the nucleic acid sequences coding for said peptides and enabling their production by genetic engineering.

This application is a 371 of PCT/FR98/01583, filed Jul. 20, 1998.

The invention relates to new antimicrobial peptides produced by penaeid prawns.

Peptides endowed with antimicrobial properties are produced by a wide variety of (animal or plant) species in which they participitate in nonspecific mechanisms of defence against infections. These peptides are the subject of increasing interest, in particular because they generally possess a broad activity spectrum and a low cytotoxicity for eukaryotic cells.

Comparisons of the amino acid sequences, of the secondary structures and the functional similarities have made it possible to define four main groups to which most of the antimicrobial peptides described up until now can be attached. For a review, cf. for example HOFFMANN et al., in Phylogenetic Perspectives in Immunity; The Insect Host Defense. Chapter 4, pp. 43-65 (1994):

1. A first group comprises linear peptides consisting essentially of basic and hydrophobic amino acids; the cecropins of insects and of mammals, and the magainins of the skin of batrachians, in particular are classified in this group;

2. A second group comprises peptides comprising intramolecular disulphide bridges; the defensins of insects or of mammals, the brevinins of the skin of batraciens, the thanatin of insects, which exhibits a strong sequence homology with the brevinins [FEHLBAUM et al. Proc. Natl. Acad. Sci. USA. 93, pp. 1221-1225, (1996)], the tachyplesins, produced by primitive marine arthropods, as well as various peptides obtained from the haemolymph of scorpions [EHRET-SABATIER et al., J. Biol. Chem., 271, 47, pp. 29537-29544, (1996)], in particular are classified in this group;

3. The third group comprises the peptides rich in proline, among which there may be classified the apidaecins and the abaecins of hymenopterans, drosocin and pyrrhocoricin, also produced by insects, and the bactenecins of mammals. The only antimicrobial peptide produced by a decapod crustacean which had been characterized up until now also belongs to this class: it is an antibacterial peptide rich in proline, similar to bactenecin-7, and obtained from the haemocytes of the crab Carcinus maenas [SCHNAPP et al., Eur. J. Biochem. 240, pp. 532-539 (1996)];

4. The fourth class comprises peptides or polypeptides rich in glycine, such as attacins, sarcotoxins and diptericins, all isolated from insects.

The inventors have now purified and characterized new antimicrobial peptides from the haemolymph of penaeid prawns, and have also obtained DNA sequences encoding these peptides.

The subject of the present invention is these antimicrobial peptides, called hereinafter penaeidins, which possess the following characteristics:

their molecular mass is about 5 to 7 kDa;

their pHi is greater than or equal to 9;

their N-terminal sequence comprises a region (A) of about 15 to 25 amino acids, rich in proline, (at least ⅛ and preferably between ⅛ and ⅓ of the amino acids of this region are prolines);

their C-terminal portion comprises a region (B) of about 20 to 30 amino acids, which contains 6 cysteine residues forming three intramolecular disulphide bridges.

According to a preferred embodiment of the present invention, region (A) comprises the following sequence (I):

Pro Xaa₁ ProXaa₂ ProXaa₃ Pro(I, SEQ ID NO: 15 or 19)

in which Pro represents a proline, Xaa₁, represents a nonpolar neutral amino acid, Xaa₂ represents a basic amino acid, Xaa₃ represents a proline or a peptide bond.

According to a preferred feature of this embodiment, region (A) comprises the following sequence (II):

ProXaa₁ ProXaa₂ ProXaa₃ ProXaa₁ Xaa₁ Xaa₂ ProXaa₄ Xaa₄ (II, SEQ ID NO: 17 or 21)

in which Pro, Xaa₁, Xaa₂ and Xaa₃ are as defined above, and Xaa4 represents a nonpolar neutral amino acid or a proline.

Advantageously, region (A) comprises the following sequence (III):

ProXaa₁ ProXaa₂ ProXaa₃ ProXaa₁ Xaa₁ Xaa₂ ProXaa₁ Pro Xaa₁ Xaa₁ ProXaa₁ Xaa₁ Pro(III, SEQ ID NO: 18 or 22)

in which Pro, Xaa₁, Xaa₂ and Xaa₃ are as defined above.

According to another preferred embodiment of the present invention, the 6 cysteine residues of region (B) are arranged according to the following sequence (IV):

Cys S₁ Cys S₂ Cys Cys S₃ Cys Cys (IV, SEQ ID NO: 16 or 20)

in which Cys represents a cysteine, S₁ represents an amino acid or a peptide sequence of 2 or 3 amino acids, S₂ represents a peptide sequence of 10 amino acids, S₃ represents a peptide sequence of 5 amino acids.

The present invention also encompasses peptides comprising or consisting of fragments of at least 5 amino acids of a penaeidin as defined above, and in particular peptides comprising or consisting of region (A) and/or region (B), as well as peptides comprising or consisting of sequences (I), (II), (III) and/or (IV).

According to a preferred embodiment of a peptide in accordance with the invention, its N-terminal end is blocked by a pyroglutamic acid residue, and/or its C-terminal end is amidated.

By way of illustration of the subject of the present invention, the characteristics of 3 penaeidins isolated from the haemolymph of the prawn Penaeus vannamei, called hereinafter penaeidin 1, penaeidin 2 and penaeidin 3, are more specifically indicated below.

“The sequences of these 3 peptides (1-letter code) are represented in FIGS. 1a (penaeidin 1, SEQ ID NO: 5), 1 b (penaeidin 2, SEQ ID NO: 6 with amidated C-terminus) and 1 c (penaeidin 3, SEQ ID NO: 7 with amidated C-terminus and pyroglutamic acid at the N-terminus); the alignment of the 3 sequences is represented in FIG. 1d: the conserved sequences are delimited. FIG. 2 represents a cDNA sequence of penaeidin 2 and the corresponding peptide sequence (3-letter code); FIG. 3A represents a cDNA sequence of penaeidin 3 and the corresponding peptide sequence (3-letter code); FIG. 3B represents the peptide sequence (1-letter code) of other isoforms of penaeidin 3 (P3-b and P3-c, SEQ ID NOS: 23 and 24, respectively): the sequence variations are delimited.”

The cDNA sequences of penaeidin 2 and of penaeidin 3 are respectively represented in the sequence listing in the annex under the numbers SEQ ID NO: 1 and SEQ ID NO:3, and the sequences of their products of translation are respectively represented in the sequence listing in the annex under the numbers SEQ ID NO: 2 and SEQ ID NO: 4.

The peptide sequences of the mature forms of penaeidins 1, 2 and 3 are respectively represented in the sequence listing in the annex under the numbers SEQ ID NO: 5, SEQ ID NO: 6 and SEQ ID NO: 7.

These penaeidins exhibit no significant homology with the antimicrobial peptides known in the prior art, and define a new group of antimicrobial peptides.

Penaeidins 1 and 2 comprise 50 amino acids, and have a molecular mass of about 5.5 kDa; penaeidin 3 comprises 62 amino acids and its molecular mass is about 6.6 kDa.

They are cationic peptides possessing a net positive charge of 7 for penaeidins 1 and 2, and of 8 for penaeidin 3; their calculated isoelectric points vary from 9.34 for penaeidins 1 and 2, to 9.84 for penaeidin 3.

The 3 peptides have an N-terminal domain rich in proline, and a C-terminal domain comprising 6 cysteine residues forming three intramolecular disulphide bridges, and where the 4 cysteine residues closest to the C-terminal end are organized into 2 doublets separated by 5 residues.

The most central cysteines are respectively separated by 1, 2, or 3 residues in penaeidins 1, 2 and 3.

The N-terminal end of penaeidin 3 is blocked by a pyroglutamic acid residue. Blocking by similar residues has already been observed in other antimicrobial peptides.

The C-terminal end of penaeidins 2 and 3 is amidated. Such a C-terminal amidation has already been observed for certain antimicrobial peptides of other marine invertebrates (Limulus tachyplesins), as well as for the cecropins of insects and the magainins of amphibians. Such a modification reinforces the stability of the molecule, and it appears that it increases the antimicrobial activity.

Penaeidins possess a high stability and are in particular highly resistant to proteolysis. They are essentially active against Gram-positive bacteria, and also exhibit activity against Gram-negative bacteria; they possess, in addition, fungicidal properties.

Penaeidins and their fragments, as defined above, may be obtained, for example, by extraction from animals producing them, and also by peptide synthesis, or advantageously by genetic engineering, by expressing at least one nucleic acid sequence encoding a penaeidin or a fragment thereof in an appropriate host cell.

The present invention also encompasses nucleic acids comprising a segment of at least 10 bp, and preferably at least 18 bp of the gene for a penaeidin.

Nucleic acids in accordance with the invention, and in particular cDNAs for penaeidin, or portions thereof, may be obtained by screening nucleic acid libraries with the aid of oligonucleotides derived from the sequences represented in FIGS. 1, 2 or 3, or their complementary sequences.

Nucleic acids in accordance with the invention also include expression cassettes comprising at least one nucleic acid sequence encoding a penaeidin or a fragment thereof, under transcriptional control of an appropriate promoter.

“Appropriate promoter” is understood to mean any promoter which is functional in the host cell designated to harbour the expression cassette.

An expression cassette in accordance with the invention may also comprise, in addition, one or more nucleic acid sequence(s) which make it possible to enhance the secretion of the penaeidin or of its fragment by the host cell, for example a sequence encoding a signal peptide. The nucleic acid sequence encoding the signal peptide is placed at the 5′ end of the nucleic acid sequence encoding the penaeidin or its fragment.

The nucleic acid sequence encoding the signal peptide may be a sequence encoding a penaeidin signal peptide, or alternatively a sequence encoding a heterologous signal peptide. A sequence will be chosen which comprises, at the C-terminal end, a site for proteolysis capable of being recognized by a signal peptidase of the host cell designated to harbour the expression cassette.

The subject of the invention is also:

recombinant vectors characterized in that they comprise at least one nucleic acid in accordance with the invention, and in particular the vectors comprising an expression cassette as defined above;

prokaryotic or eukaryotic cells transformed with an expression cassette according to the invention. They may be cells maintained in culture, or cells forming part of a pluricellular, animal or plant, organism. The expression cassette present in the transformed cell may be either incorporated into the chromosomal DNA of the said cell, or may be carried by an extra chromosomal vector.

The subject of the invention is also a method of producing a penaeidin or a fragment thereof, characterized in that it comprises the expression of the said penaeidin or of the said fragment in at least one transformed cell in accordance with the invention.

The peptides in accordance with the invention may be expressed in cultures of transformed cells using techniques similar to those used for antimicrobial peptides of the prior art, for example in insect cells, as described by HELLERS et al. [Eur. J. Biochem. 199, pp. 435-439, (1991)] for cecropins, or in yeast, as described by REICHHART et al. [Invertebrate Reproduction and Development, 21, pp. 15-24, (1992)].

They may also be expressed in transgenic plants or animals, in order to increase the resistance of the latter to infections, as described for example by JAYNES et al., [Plant Science, 89, pp. 43-53 (1993)] in the case of peptides which are analogues of cecropin B, expressed in transgenic tobacco plants, or by NORELLI et al. [Euphytica, 77, pp. 123-128 (1994)] for transgenic young apple trees expressing the gene of attacin-E.

The peptides in accordance with the invention can be used in particular for the production of products and in particular of medicaments against infections, for example antibacterials or fungicides.

Such products find their application for the prevention and the treatment of various microbial diseases, in a very wide variety of sectors, in particular in the fields of health and agriculture, and in that of aquaculture, for limiting the development of infectious diseases in breeding farms.

The present invention will be understood more clearly with the aid of the additional description which follows, which refers to examples of purification of the antimicrobial peptides in accordance with the invention, and their characterization.

EXAMPLE 1 ISOLATION OF ANTIMICROBIAL PEPTIDES FROM THE PRAWN PENAEUS VANNAMEI

225 ml of haemolymph were obtained from 500 prawns, by collecting samples from the central sinus, situated at the base of the first abdominal segment. The collection of sample was made in the presence of anticoagulant buffer (10% sodium citrate, pH 7, supplemented with 200 μM of phenylthiourea and 40 μg/ml of aprotinin. The haemolymph was then centrifuged at 700 g for 15 minutes at 4° C. in order to remove the blood cells. The plasma and the haemocytes are stored separately at −70° C. up to their use.

1.1 Preparation of the Fractions of the Haemolymph by Acid Extraction

a) Plasma:

The plasma is diluted with demineralized water (1 volume of plasma per 1 volume of water), and 0.1% (v/v) of trifluoroacetic acid is added to this mixture. The pH was brought to 3.9 with the aid of 1 M HCl in a bath cooled with ice, with gentle stirring for 1 hour. Two successive centrifugations at 8000 g and at 4° C. were carried out in order to clarify the supernatant which is stored on ice at 4° C. up to its use.

b) Haemocytes:

Cytosol

After thawing, the haemocytes are homogenized with the aid of a DOUNCE apparatus (maximum 152 μ, minimum 76 μ) in 50 mM Tris buffer, pH 8.7, containing 50 mM NaCl. After centrifugation (8000 g, 20 min, 4° C.), the supernatant (cytosol fraction) is acidified to a pH of 3.6 by addition of 1 M HCl and stored at 4° C.

Organelles

The pellet containing the cellular organelles is extracted by sonication (3-30 s) at medium power in 2 M acetic acid. The debris is removed by centrifugation (8000 g, 20 min, 4° C.) and the acidic extract stored at 4° C. up to its use.

1.2 Purification of the Peptides

A) Solid phase extraction

The plasma fraction and the cytosol and organelle extracts were deposited separately on 35 cm³ SEP-PAK C₁₈ VAC cartridges (10 g WATERS ASSOCIATES), equilibrated with acidified water (0.05% of trifluoroacetic acid).

After washing with acidified water, 3 elutions are carried out using successively 5%, 40% and 80% acetonitrile solutions in acidified water. The different fractions obtained are lyophilized and reconstituted with microfiltered water prior to purification by reversed-phase HPLC.

B) HPLC purification

1: Reversed-phase HPLC.

The fractions eluted at 5% and 40% of the SEP-PAK column were subjected to a reversed-phase chromatography on an AQUAPORE RP300 C₈ column (4.6×220 mm, BROWNLEE™), equilibrated in acidified water (0.05% TFA). The separation of the 5% SEP-PAK fraction is carried out with the aid of a linear gradient of 0 to 5% acetonitrile in acidified water, at a flow rate of 1 ml/min for 80 min.

For the 40% fraction, a linear gradient of 2 to 60% acetonitrile is used under the same conditions. The fractions are collected, dried under vacuum, reconstituted with the aid of ultrafiltered water, and their antimicrobial activity is tested.

2: Exclusion chromatography.

The fractions obtained at the end of the reversed-phase chromatography, exhibiting antimicrobial activity, are purified by exclusion chromatography using 2 serially linked HPLC columns (ULTRASPHEROGEL SEC 3000 column and ULTRASPHEROGEL SEC 2000 column, 7.5×300 mm, BECKMAN) protected by a precolumn (ULTRASPHEROGEL SEC, 7.5×40 mm, BECKMAN).

The elution is carried out under isocratic conditions in the presence of 30% acetonitrile in acidified water, at a flow rate of 0.5 ml/min. The fractions are collected and tested for their antimicrobial activity.

3: Reversed-phase chromatography

The peptides were purified by reversed-phase chromatography on the same column as in step 1 at a controlled temperature of 35° C.

The peptides 1 and 2 were purified with a linear biphasic gradient: of 2 to 21% acetonitrile in acidified water (0.05% TFA) over 10 min, and then of 21 to 35% acetonitrile over 50 min at a flow rate of 0.25 ml/min.

Peptide 3 was purified with the aid of a linear biphasic gradient: of 2 to 23% acetonitrile in acidified water over 10 min, and then of 23 to 37% acetonitrile over 50 min at a flow rate of 0.25 ml/min at 35° C.

4: Last purification steps

The last purification steps for the peptides were performed on DELTA PAK HPI C₁₈ reversed-phase column, 2×150 mm, WATERS ASSOCIATES, at a temperature of 40° C. with a flow rate of 0.25 ml/min using the biphasic gradients described above.

The elution is monitored by ultraviolet absorption at 225 nm. The fractions are collected in polypropylene tubes, concentrated under vacuum and reconstituted with water sterilized by filtration prior to the measurement of the antimicrobial activity.

EXAMPLE 2 ANTIMICROBIAL ACTIVITY OF THE FRACTIONS OBTAINED

The 40% SEP PAK fractions obtained from the plasma extract, from the organelles of the haemocytes, and from the cytosolic fraction, which are respectively designated hereinafter by the names P40, HO40 and HC40, were mainly studied. All the fractions were tested for their activity against 2 bacterial strains and against a filamentous fungus.

Microorganisms:

The microbial strains used to determine the antimicrobial activities are the following:

Micrococcus luteus (Gram-positive strain);

Escherichia coli (Gram-negaitve strain);

Neurospora crassa (filamentous fungus).

The bacteria are cultured with a starting optical density A₆₀₀=0.001 in “Poor-Broth” nutrient medium (1% bactotryptone, 0.5% NaCl w/v).

Antibacterial assays:

The fractions were tested by inhibition of growth in liquid phase.

10 μl aliquots of each fraction to be tested are incubated in microtitre plates with 100 μl of a suspension of bacteria at the midlogarithmic phase.

The bacterial growth is measured by determining the optical density A₆₀₀ after an incubation of 24 hours at 30° C.

An identical procedure was used to determine the minimum inhibitory concentration (MIC) of the molecules on these bacterial strains. The MIC values correspond to an interval a-b of peptide concentrations, where a represents the highest concentration at which bacterial growth is observed, and b is the lowest concentration which causes 100% inhibition of growth.

Measurement of the bacteriostatic properties

A midlogarithmic phase culture of Micrococcus luteus in the “Poor Broth” medium defined above was incubated at 30° C. in the presence of the peptide or (as control) of water. The final concentration of the molecules tested is 8 times higher than the MIC. 20-μl alquots are collected at various time intervals and cultured on nutrient agar plate. The number of colonies formed (CFU) is determined after 24 hours of incubation at 37° C.

Antifungal activity

The antifungal activity was determined against Neurospora crassa and Fusarium oxysporum by a test of inhibition of growth in liquid phase.

80 μl of fungal spores (final concentration: 10⁴ spores/ml) are suspended in ½ Potato Dextrose Broth (DIFCO) supplemented with tetracycline (10 μg/ml) and cefotaxime (100 μg/ml). This suspension was added to 10 μl of the fraction to be tested in microtitre plates. The final volume is brought to 100 μl by addition of 10 μl of water. Growth inhibition can be observed under a microscope after 24 hours of incubation at 25° C. in the dark, and measured by the variation in the optical density at 600 nm after 48 hours.

No activity was obtained for the fractions obtained from HC40. On the other hand, as shown respectively in FIGS. 4 and 5, some of the fractions obtained from P40 and HO40 exhibit antimicrobial activity and possess a common active zone corresponding to the fractions eluted between 47 and 60 min (26 to 29% acetonitrile), where activity is observed against the 2 bacterial strains and the fungus.

Legend to FIGS. 4 and 5:

The elution time in min is indicated on the x-axis. The absorbance at 225 nm is represented on the y-axis. The antimicrobial activity against Escherichia coli D31 (hashed rectangle), Micrococcus luteus (black rectangle) and N. crassa (grey rectangle) is indicated for the relevant fractions. The regions of the curve corresponding to the fractions exhibiting antimicrobial activity are indicated by the letters A, B and C. The fractions from which penaeidins 1, 2 and 3 were purified are indicated by the arrows 1, 2 and 3.

2 regions also exhibiting antimicrobial activity were obtained after reversed-phase chromatography of the plasma fraction. The P40B fractions, which are eluted at around 40 min (21-22% acetonitrile), contain molecules which are active against the 3 microorganisms tested, and P40C, corresponding to molecules eluted at around 75 min, 38-39% acetonitrile, are active only against Micrococcus luteus. Neither of these two zones is present in the chromatogram for HO40, in which other fractions eluted at about 90 min (45-46% acetonitrile) are active only against Micrococcus luteus.

EXAMPLE 3 DETERMINATION OF THE STRUCTURE OF THE PEPTIDES OBTAINED

The three peptides purified as indicated in Example 1 above were characterized by successive biochemical and molecular biology techniques.

Their molecular mass was determined by mass spectrometry; this molecular mass is respectively 5484.8 Da and 5520.0 Da for peptides P1 and P2, and 6617.4 Da for peptide P3.

It was possible to obtain the amino acid sequence of P1 by direct sequencing by Edman degradation.

P1 is a peptide of 50 residues, containing 14% of proline among the 19 N-terminal residues, and 6 cysteines forming 3 intramolecular disulphide bridges in the C-terminal domain. P1 is rich in basic amino acids, as indicated by the presence of 5 arginines and 2 lysines distributed over the entire length of the peptide.

As regards peptide P2, direct sequencing made it possible to obtain only a partial sequence of 21 N-terminal residues. This sequence differs from that of penaeidin 1 only in the replacement of leucine at position 20 in P1 with a phenylalanine in P2.

As regards peptide P3, it was not possible to establish its N-terminal sequence by the Edman degradation technique, which suggests a blocking of its N-terminal end.

S-pyridylethylation followed by mass spectrometry analysis, and the detection of the variation in mass induced by S-pyridylethylation, shows the presence of 6 cysteine residues. After enzymatic cleavage of the S-pyridylethylated peptide, the fragments obtained were purified, analysed by mass spectrometry, and by Edman degradation. The sequence of the NH₂ -terminal end was determined by nanoES-MS (nanoElectrospray Mass Spectrometry). It was thus possible to determine that the NH₂ -terminal residue of this peptide was cyclized into pyroglutamic acid.

EXAMPLE 4 CLONING OF THE CDNA FOR PENAEIDINS

cDNA for penaeidin 3:

A probe consisting of degenerate oligonucleotides corresponding to residues 38-44 of peptide P3, and having the following sequence:

5′ (GGIAT (A/T/C) (A/T) (G/C) ITT (C/T) (A/T) (G/C) ICA(A/G) GC)3′ (SEQ ID NO:1) was prepared.

This probe made it possible to obtain, by reverse transcription followed by polymerase chain reaction (PCR) starting with total poly(A)⁺ RNAs of haemocytes of adult prawns, harvested 6 and 12 hours after a bacterial challenge, an amplification product consisting of a fragment of 497 base pairs. After sequencing, this fragment was identified as a P3 cDNA fragment consisting of the end of the open reading frame and the 3′ untranslated region. A subfragment of 440 base pairs, obtained by enzymatic BsaI digestion from this fragment of 497 base pairs and consisting mainly of the 3′ unstranslated region, was cloned into a vector pBluescript (STRATAGENE, La Jolla, Calif.). This fragment was labelled by random priming using the READY-TO-GO DNA labelling kit (PHARMACIA BIOTECH, Uppsala, Sweden), and used to screen a cDNA library obtained from poly(A)±RNAs of adult prawns, in the ZAP-EXPRESS vector (STRATAGENE, La Jolla, Calif.).

High stringency hybridizations were carried out overnight at 65° C. in a solution of 5X Denhardt, 5X SSPE, 0.1% SDS, in the presence of 100 μg/μl of salmon sperm DNA.

161 positive clones were obtained. Among these clones, 4 were sequenced. One of them contained an open reading frame encoding a sequence of 81 amino acids (P3a) starting with a methionine codon and ending with a stop codon.

The deduced amino acid sequence of this open reading frame starts with a signal peptide of 19 residues rich in hydrophobic residues. The signal peptidase cleavage site was located after the glycine residue preceding the glutamine at position 1.

This signal peptide is directly followed, at its C-terminal end, by a peptide of 63 amino acids, starting with a glutamine residue. The deduced amino acid sequence of this mature peptide clearly confirms the partial sequences of penaeidin 3 which are obtained by direct sequencing.

Starting with the hypothesis that penaeidin 3 in mature form starts with a pyroglutamic acid resulting from the cyclization of the glutamine residue (observed by a biochemical approach), the mass calculated from the deduced amino acid sequence is 56.4 Da higher than the measured mass. This suggests that penaeidin 3 is amidated at its C-terminal end by elimination of a glycine residue (57 Da).

Among the other 3 clones sequenced, 2 slightly different deduced amino acid sequences were identified (P3b and P3c). P3b differs from P3a by the replacement of an isoleucine at position 30 in P3a by a valine in P3b, whereas p3c lacks, at position 33, a proline which is present in P3a and P3b. Finally, the leucine at position 40 in P3a and P3b is replaced by a valine in p3c.

CDNA for penaeidin 2:

In order to isolate the cDNAs for the other penaeldins, a probe corresponding to a portion of the coding sequence of penaeidin 3 in which the sequence of the product of translation is highly conserved between the 3 purified peptides, was used.

This probe was obtained by polymerase chain reaction on a p3 cDNA clone using the following primers:

upstream primer: 5′GTGTACAAGGGCGGTTACACG3′ (SEQ ID NO: 8).

downstream primer: 5′CAACAGGTTGTCAAGCGAGGT3′ (SEQ ID NO: 9).

The hybridization is performed under the same conditions as those described above, but at a lower hybridization temperature (50° C.) in order to ensure a lower stringency.

The clones obtained under these conditions were analysed on the basis of their restriction profile. For one of these clones, this profile was different from that observed for P3a. This clone was sequenced. The amino acid sequence deduced from the DNA sequence possesses a very high percentage identity with penaeidin 1, in particular a COOH-terminal domain which is completely identical to that established by direct sequencing of penaeidin 1. However, the presence of a phenylalanine residue at position 20 in the mature peptide, as well as the value of the molecular mass, is in favour of the cDNA for penaeidin 2. The deduced amino acid sequence comprises an additional amino acid (glycine at the C-terminal end), compared with that obtained by direct sequencing, which suggests that this glycine residue may be removed by amidation. This was confirmed by the calculation of the mass (5575.6 Da) which is 55.6 Da higher than the mass measured for penaeidin 2, whereas the calculated mass of the amidated peptide corresponds to that of penaeidin 2 measured by mass spectrometry.

It appears, in addition, that penaeidin 2 is derived from a precursor molecule possessing a “pre” region of 21 residues, which is identical to that of the signal peptide of penaeidin 3, with two additional residues (Glu-Ala) immediately preceding the observed cleavage site. Another potential cleavage site was located at position -3 upstream of the Tyr-1 residue. This other site corresponds to that which is observed in the maturation of penaeidin 3.

EXAMPLE 5 ANTIMICROBIAL ACTIVITY OF THE PURIFIED PENAEIDIN 3

The activity of the purified penaeidin 3 was tested as described in Example 2, against Micrococcus luteus, Escherichia coli D31, Neurospora crassa and also against Fusarium oxysporum which is a pathogenic fungus for penaeid prawns.

This peptide has a marked activity against Micrococcus luteus (MIC=0.6 to 2.5 μM) and moderate activity on Escherichia coli (MIC>5 μM). Penaeidin 3 is also active against the 2 fungi tested (MIC>5 μM).

When the incubation of penaeidin 3 with Micrococcus luteus is carried out at a concentration of 18 μM, that is to say 8 times higher than the MIC, no bacterial growth is observed after an incubation of 24 hours. Furthermore, the number of colonies formed remains constant during the different sample collections, which suggests that this molecule has a bacteriostatic effect. These results are illustrated by Table I below.

TABLE I Incubation time Control Penaeidin 3 10⁴ cfu/ml 1 min 3.85 4.30 30 min 4.40 4.45 2 h 6.70 3.15 4 h 9.45 4.60 7 h 51.50 4.50 24 h >1,000 5.60

EXAMPLE 6 PRODUCTION OF RECOMBINANT PENAEIDINS

The DNA fragments encoding penaeidins-2 or -3, flanked at their amino-terminal end, were obtained by PCR using the following primers, which make it possible to insert, upstream of the sequence encoding the penaeidin, a sequence encoding the last five residues of the proregion of the yeast sex factor MFαl and a HindIII site, and downstream of the sequence encoding the penaeidin, a BamHI site.

Primers used for penaeidin-2:

P2-SENSE: _(5′)GCGCGCAAGCTTGGACAAGAGATACAGGGGCGGTTAC_(3′)         Hind III P2-ANTI: _(5′)GCGCGCGGATCCTTATCCTTTTACTAAGTGACAAC_(3′)         Bam HI

These 2 primers are respectively represented in the sequence listing under the numbers SEQ ID NO: 10 and SEQ ID NO: 11.

Primers used for penaeidin-3:

P3-SENSE: _(5′)GCGCGCAAGCTTGGACAAGAGACAAGTGTACAAGGGC_(3′)         Hind III P3-ANTI: _(5′)GCGCGCGGATCCTCAAACCGGAATATCC_(3′)         Bam HI

These 2 primers are respectively represented in the sequence listing under the numbers SEQ ID NO: 12 and SEQ ID NO: 13.

The enzyme used for the PCR is Vent polymerase (NEW ENGLAND BIOLABS) which generates blunt ends. The fragments obtained were cloned between the HindIII and BamHI sites of the multisite linker of the plasmid pCR-SCRIPT (STRATAGENE).

The inserts of the plasmids thus obtained were excised with BamII and HindIII, and subcloned into the yeast expression cassette of the vector pJV1. The vector pJV1 was provided by the laboratory of Prof. HOFFMANN at Strasbourg: it is a plasmid derived from pBLUESCRIPT II into which there have been introduced the promoter of the yeast sex factor MFαl and the preprosequence of this same factor, in which sequence there has been created, by silent mutation, a HindIII restriction site allowing the fusion in phase of the coding sequences.

The complete cassettes (about 1.4 kb) were excised from pJV1 by double SphI-BamHI digestion (partial SphI digestion for penaeidin-2) and subcloned into the shuttle vector pTG4812 [MICHAUD et al. FEBS Lett., 395: 6-10 (1996)].

The two expression vectors thus obtained are called pen2-pTG4812 and pen3-pTG4812, and allow respectively the expression of penaeidin 2 and that of penaeidin 3 in yeast.

For the production of recombinant penaeidins, the Saccharomyces cerevisiae TGY48-1 strain, described by REICHHART et al. [Invert. Reprod. Dev. 21(1): 15-24 (1992)], was used. It carries a ura3-Δ5 mutation which allows the selection of recombinant clones on uracil-free selective medium.

This strain was transformed with the recombinant plasmids pen2-pTG4812 and pen3-pTG4812 according to the lithium acetate method described by GIETZ et al. [Nucl. Ac. Res. 20(6): 1425 (1992)]. The recombinant yeast clones were selected on uracil-free selective medium “YNBG-casamino acids” and cultured. The recombinant penaeidins are secreted into the culture medium and are purified from the culture supernatant according to a protocol similar to that described in Example 1 above.

24 1 658 DNA Penaeus vannamei CDS (76)..(294) mat_peptide (139)..(294) 1 aattcggcac gagctccctc tagcctcacc tgcagagacc gacgctccga gcccgggttc 60 cctcctgcgt ccgcc atg cgc ctc gtg gtc tgc ctg gtc ttc ttg gcc tcc 111 Met Arg Leu Val Val Cys Leu Val Phe Leu Ala Ser -20 -15 -10 ttc gcc ctg gtc tgc caa ggc gaa gcg tac agg ggc ggt tac aca ggc 159 Phe Ala Leu Val Cys Gln Gly Glu Ala Tyr Arg Gly Gly Tyr Thr Gly -5 -1 1 5 ccg ata ccc agg cca cca ccc att gga aga cca ccg ttc aga cct gtt 207 Pro Ile Pro Arg Pro Pro Pro Ile Gly Arg Pro Pro Phe Arg Pro Val 10 15 20 tgc aat gca tgc tac aga ctt tcc gtc tca gat gct cgc aat tgc tgc 255 Cys Asn Ala Cys Tyr Arg Leu Ser Val Ser Asp Ala Arg Asn Cys Cys 25 30 35 atc aag ttc gga agc tgt tgt cac tta gta aaa gga taa agaaattgac 304 Ile Lys Phe Gly Ser Cys Cys His Leu Val Lys Gly 40 45 50 ggagaagaca atggaaacct ggcttgacaa cttgttaatt aatactcata tgtgaagaga 364 ttgcaaccct gatttgtgtc aaggatgtgg gtatttcgtc tatccatcgc taaagattct 424 tccatgaatg tatgatgaag gaaagtgcat gtgtgtaagt atgtatgtat gtgcttacag 484 gtatttgttg cattaagtgt ccgtgtattt aggatctgca acacacgagg aagagaatat 544 ttgccacttg ccatttattt cagtttctgt aagtgtggat ctgtgagagg ttggtgttga 604 cagatctctc ttttacaaat aaatttgata tctgtgaaaa aaaaaaaaaa aaaa 658 2 72 PRT Penaeus vannamei 2 Met Arg Leu Val Val Cys Leu Val Phe Leu Ala Ser Phe Ala Leu Val 1 5 10 15 Cys Gln Gly Glu Ala Tyr Arg Gly Gly Tyr Thr Gly Pro Ile Pro Arg 20 25 30 Pro Pro Pro Ile Gly Arg Pro Pro Phe Arg Pro Val Cys Asn Ala Cys 35 40 45 Tyr Arg Leu Ser Val Ser Asp Ala Arg Asn Cys Cys Ile Lys Phe Gly 50 55 60 Ser Cys Cys His Leu Val Lys Gly 65 70 3 736 DNA Penaeus vannamei CDS (71)..(319) mat_peptide (128)..(319) 3 aattcggcac gagtcgagcc tcacctgcag agaccgacgc tccgagcccg ggttccctcc 60 tgcgtccgcc atg cgc ctc gtg gtc tgc ctg gtc ttc ttg gcc tcc ttc 109 Met Arg Leu Val Val Cys Leu Val Phe Leu Ala Ser Phe -15 -10 gcc ctg gtc tgc caa ggc caa gtg tac aag ggc ggt tac acg cgc ccg 157 Ala Leu Val Cys Gln Gly Gln Val Tyr Lys Gly Gly Tyr Thr Arg Pro -5 -1 1 5 10 ata ccc agg cca cca ccc ttc gtg aga cct ttg cca gga ggg cct att 205 Ile Pro Arg Pro Pro Pro Phe Val Arg Pro Leu Pro Gly Gly Pro Ile 15 20 25 ggt cca tac aac ggt tgc cct gtc tca tgc cgg gga att tcc ttc tca 253 Gly Pro Tyr Asn Gly Cys Pro Val Ser Cys Arg Gly Ile Ser Phe Ser 30 35 40 caa gcg cgt tct tgc tgc tcc cgg tta ggg cgt tgc tgt cac gtg gga 301 Gln Ala Arg Ser Cys Cys Ser Arg Leu Gly Arg Cys Cys His Val Gly 45 50 55 aag gga tat tcc ggt tga tggagaacac gatgaaaacc tcgcttgaca 349 Lys Gly Tyr Ser Gly 60 acctgttgat tgatacttgt atgtgaagag actgtgatcc tgattttgca ctgtgttttc 409 tcgttcaata ttcttactct ggcttgtgga atggatgtag ttatttgacc ctatgttttt 469 ttttaagact tttccatgaa tgcacgatga atgaaagctt gcgtgatatg aatgagtgca 529 tccacttttc aacgtcccag caggtggcgc cgtattcatg atttgtgaca cacgaggaag 589 taaatccatg ccatctgcct ttcgttgtaa tttttagtga gtatggatct gtgtgtgggt 649 gatttttaca aatctctcaa aggactttta gaaatgttac tcctttacaa ataaaattgg 709 tatcttgaaa aaaaaaaaaa aaaaaaa 736 4 82 PRT Penaeus vannamei 4 Met Arg Leu Val Val Cys Leu Val Phe Leu Ala Ser Phe Ala Leu Val 1 5 10 15 Cys Gln Gly Gln Val Tyr Lys Gly Gly Tyr Thr Arg Pro Ile Pro Arg 20 25 30 Pro Pro Pro Phe Val Arg Pro Leu Pro Gly Gly Pro Ile Gly Pro Tyr 35 40 45 Asn Gly Cys Pro Val Ser Cys Arg Gly Ile Ser Phe Ser Gln Ala Arg 50 55 60 Ser Cys Cys Ser Arg Leu Gly Arg Cys Cys His Val Gly Lys Gly Tyr 65 70 75 80 Ser Gly 5 50 PRT Penaeus vannamei 5 Tyr Arg Gly Gly Tyr Thr Gly Pro Ile Pro Arg Pro Pro Pro Ile Gly 1 5 10 15 Arg Pro Pro Leu Arg Leu Val Val Cys Ala Cys Tyr Arg Leu Ser Val 20 25 30 Ser Asp Ala Arg Asn Cys Cys Ile Lys Phe Gly Ser Cys Cys His Leu 35 40 45 Val Lys 50 6 50 PRT Penaeus vannamei 6 Tyr Arg Gly Gly Tyr Thr Gly Pro Ile Pro Arg Pro Pro Pro Ile Gly 1 5 10 15 Arg Pro Pro Phe Arg Pro Val Cys Asn Ala Cys Tyr Arg Leu Ser Val 20 25 30 Ser Asp Ala Arg Asn Cys Cys Ile Lys Phe Gly Ser Cys Cys His Leu 35 40 45 Val Lys 50 7 62 PRT Penaeus vannamei 7 Glu Val Tyr Lys Gly Gly Tyr Thr Arg Pro Ile Pro Arg Pro Pro Pro 1 5 10 15 Phe Val Arg Pro Leu Pro Gly Gly Pro Ile Gly Pro Tyr Asn Gly Cys 20 25 30 Pro Val Ser Cys Arg Gly Ile Ser Phe Ser Gln Ala Arg Ser Cys Cys 35 40 45 Ser Arg Leu Gly Arg Cys Cys His Val Gly Lys Gly Tyr Ser 50 55 60 8 21 DNA Penaeus vannamei PCR primer 8 gtgtacaagg gcggttacac g 21 9 21 DNA Penaeus vannamei PCR primer 9 caacaggttg tcaagcgagg t 21 10 37 DNA Artificial Sequence PCR primer 10 gcgcgcaagc ttggacaaga gatacagggg cggttac 37 11 35 DNA Artificial Sequence PCR primer 11 gcgcgcggat ccttatcctt ttactaagtg acaac 35 12 37 DNA Artificial Sequence PCR primer 12 gcgcgcaagc ttggacaaga gacaagtgta caagggc 37 13 28 DNA Artificial Sequence PCR primer 13 gcgcgcggat cctcaaaccg gaatatcc 28 14 20 DNA Artificial Sequence RT-PCR primer for cloning penaeidin-3 14 ggnathwsnt tywsncargc 20 15 7 PRT Artificial Sequence Penaeidin N-terminal motif 15 Pro Xaa Pro Xaa Pro Xaa Pro 1 5 16 23 PRT Artificial Sequence Description of Artificial Sequence Penaeid C-terminal motif 16 Cys Xaa Xaa Cys Xaa Xaa Xaa Xaa Xaa Xaa Xaa Xaa Xaa Xaa Cys Cys Xaa Xaa 1 5 10 15 Xaa Xaa Xaa Cys Cys 20 17 13 PRT Artificial Sequence Description of Artificial Sequence Penaeid N-terminal motif 17 Pro Xaa Pro Xaa Pro Xaa Pro Xaa Xaa Xaa Pro Xaa Xaa 1 5 10 18 19 PRT Artificial Sequence Description of Artificial Sequence Penaeid N-terminal motif 18 Pro Xaa Pro Xaa Pro Xaa Pro Xaa Xaa Xaa Pro Xaa Pro Xaa Xaa Pro 1 5 10 15 Xaa Xaa Pro 19 6 PRT Artificial Sequence Penaeidin N-terminal motif 19 Pro Xaa Pro Xaa Pro Pro 1 5 20 24 PRT Artificial Sequence Description of Artificial Sequence Penaeid C-terminal motif 20 Cys Xaa Xaa Xaa Cys Xaa Xaa Xaa Xaa Xaa Xaa Xaa Xaa Xaa Xaa Cys Cys Xaa 1 5 10 15 Xaa Xaa Xaa Xaa Cys Cys 20 21 12 PRT Artificial Sequence Description of Artificial Sequence Penaeid N-terminal motif 21 Pro Xaa Pro Xaa Pro Pro Xaa Xaa Xaa Pro Xaa Xaa 1 5 10 22 18 PRT Artificial Sequence Description of Artificial Sequence Penaeid N-terminal motif 22 Pro Xaa Pro Xaa Pro Pro Xaa Xaa Xaa Pro Xaa Pro Xaa Xaa Pro 1 5 10 15 Xaa Xaa Pro 23 82 PRT Penaeus vannamei Penaeidin 3 isoform 23 Met Arg Leu Val Val Cys Leu Val Phe Leu Ala Ser Phe Ala Leu Val 1 5 10 15 Cys Gln Gly Gln Val Tyr Lys Gly Gly Tyr Thr Arg Pro Val Pro Arg 20 25 30 Pro Pro Pro Phe Val Arg Pro Leu Pro Gly Gly Pro Ile Gly Pro Tyr 35 40 45 Asn Gly Cys Pro Val Ser Cys Arg Gly Ile Ser Phe Ser Gln Ala Arg 50 55 60 Ser Cys Cys Ser Arg Leu Gly Arg Cys Cys His Val Gly Lys Gly Tyr 65 70 75 80 Ser Gly 24 81 PRT Penaeus vannamei Penaeidin 3 isoform 24 Met Arg Leu Val Val Cys Leu Val Phe Leu Ala Ser Phe Ala Leu Val 1 5 10 15 Cys Gln Gly Gln Val Tyr Lys Gly Gly Tyr Thr Arg Pro Ile Pro Arg 20 25 30 Pro Pro Phe Val Arg Pro Val Pro Gly Gly Pro Ile Gly Pro Tyr Asn 35 40 45 Gly Cys Pro Val Ser Cys Arg Gly Ile Ser Phe Ser Gln Ala Arg Ser 50 55 60 Cys Cys Ser Arg Leu Gly Arg Cys Cys His Val Gly Lys Gly Tyr Ser 65 70 75 80 Gly 

What is claimed is:
 1. A purified antimicrobial peptide obtained from penaeid prawns with a molecular mass of about 5 to 7 kDa and an isoelectric pH greater than or equal to 9; said peptide comprising: a) a N-terminal region (A) of about 15 to 25 amino acids, which comprises the following motif (I): ProXaa₁ ProXaa₂ ProXaa₃ Pro(SEQ ID NO:15 or 19) in which Xaa₁ is a nonpolar neutral amino acid, Xaa₂ is a basic amino acid and Xaa₃ is a proline or a peptide bond; and b) a C-terminal region (B) of about 20 to 30 amino acids, which contains 6 cysteine residues forming three intramolecular disulfide bridges and arranged according to the following motif (IV): Cys S₁ Cys S₂ Cys Cys S₃ Cys Cys (SEQ ID NO: 16 or 20) in which S₁ is an amino acid or a peptide sequence of 2 or 3 amino acids, S₂ is a peptide sequence of 10 amino acids, and S₃ is a peptide sequence of 5 amino acids.
 2. The peptide according to claim 1, in which region (A) comprises the following sequence (II): ProXaa₁ ProXaa₂ ProXaa₃ ProXaa₁ Xaa₁ Xaa₂ ProXaa₄ Xaa₄ (SEQ ID NO:17 or 21) in which Xaa₁, Xaa₂, and Xaa₃ are as defined in claim 1, and Xaa4 is a nonpolar neutral amino acid or a proline.
 3. The peptide according to claim 1, in which region (A) comprises the following sequence (III): ProXaa₁ ProXaa₂ ProXaa₃ ProXaa₁ Xaa₁ Xaa₂ ProXaa₁ ProXaa₁ Xaa₁ ProXaa₁ Xaa₁ Pro (SEQ ID NO: 18 or 22) in which Xaa₁, Xaa₂, and Xaa₃ are as defined in claim
 1. 4. The peptide according to claim 1, selected from the group consisting of: the peptide having the sequence of SEQ ID NO: 5, the peptide having the sequence of SEQ ID NO: 6, and the peptide having the sequence of SEQ ID NO:
 7. 5. The peptide according to claim 1, in which the N-terminal end is blocked by a pyroglutamic acid residue.
 6. A method of producing an antimicrobial peptide obtained from penaeid prawns with a molecular mass of about 5 to 7 kDa and an isoelectric pH greater than or equal to 9, wherein the peptide comprises: a) a N-terminal region (A) of about 15 to 25 amino acids, which comprises the following motif (I): Pro Xaa₁ Pro Xaa₂ Pro Xaa₃ Pro (SEQ ID NO:15 or 19), in which Xaa₁ is a nonpolar neutral amino acid, Xaa₂ is a basic amino acid and Xaa₃ is a proline or a peptide bond; and b) a C-terminal region (B) of about 20 to 30 amino acids, which contains 6 cysteine residues forming three intramolecular disulfide bridges and which comprises the following motif (IV): Cys S₁ Cys S₂ Cys Cys S₃ Cys Cys (SEQ ID NO: 16 or 20), in which S₁ is an amino acid or a peptide sequence of 2 or 3 amino acids, S₂ is a peptide sequence of 10 amino acids and S₃ is a peptide sequence of 5 amino acids, the method comprising: expressing a nucleic acid sequence encoding the peptide in a host cell transformed with a recombinant vector containing the nucleic sequence under the transcriptional control of an appropriate promoter, wherein the peptide is expressed in the host cell.
 7. The method of claim 6, further comprising recovering the peptide from the host cell.
 8. The method of claim 6, wherein the nucleic acid sequence encoding the peptide is obtained by screening a nucleic acid library from a penaeid prawn with at least one oligonucleotide, or the complement thereof, of at least 20 contiguous base pairs obtained from a sequence encoding a peptide selected from the group consisting of SEQ ID NO: 5, SEQ ID NO: 6 and SEQ ID NO:
 7. 9. The peptide according to claim 1, in which the C-terminal end is amidated. 